Typically, such apparatuses for blow-molding plastic containers comprise blow-molding stations that define one or more molding cavities for forming the containers.
In particular, the blow-molding stations comprise two lateral bodies, and a respective bottom for each forming cavity defined by the blow-molding station.
At least one of the lateral bodies is movable on command by way of cyclic opening and closing means, as is the bottom, so as to allow the transition of the forming station from an open condition, in which the preform or preforms can be fed in and the blow-molded container can be ejected, and a closed condition, in which the blow-molding of the preform is carried out in order to obtain the container.
During blow-molding operations, it is necessary to evacuate the air from the mold, as this air hampers the correct forming of the container.
Such evacuation is usually achieved by providing air evacuation ducts that lead into the mold.
Such solution, while conceptually valid, has some drawbacks.
In particular, the efficacy and speed of evacuation of the air is proportional to the so called total vent cross-section area, which corresponds to the sum of the areas of the holes of the evacuation ducts.
For this reason it would be preferable to have a high number of evacuation ducts but, for reasons of construction, it is not always possible to position them where they are needed without blocking the cooling circuit.
In order to increase the total vent cross-section area without compromising the quality of the molding, a solution has been proposed, in French patent application no. FR2947755 in the name of Comep, which entails providing the bottom in two parts.
In particular, the two parts comprise an annular external body that defines a plurality of recesses extending in a radial direction and mutually angularly spaced apart, and an internal body that has a plurality of complementarily-shaped raised portions designed to engage in corresponding recesses.
The internal body is therefore locked with respect to the external body, so that between the two bodies an annular evacuation opening is provided that extends around the axis of the bottom and which is connected to one or more air evacuation ducts.
Such solution, while conceptually valid, has some drawbacks, however.
First of all, it should be noted that in the design phase, the corresponding optimal positioning between the internal body and the external body needs to be studied as a function of the shape of the container to be molded.
This makes the use of the solution described rather inflexible, since the bottom furthermore needs to be redesigned if it is necessary to reduce or increase, even slightly, the internal volume of the container to be obtained.